Ted Cruz’s private life has come under renewed scrutiny amid fresh allegations that his telephone number was listed in a "black book" of contacts kept by a Washington madam who provided prostitutes to politicians.

The claim adds a fresh twist to recent allegations that the Republican presidential candidate - locked in a battle with Donald Trump for his party’s nomination - cheated on his wife Heidi by having five mistresses.

Mr Cruz, who had previously dismissed those suggestions as “complete garbage and lies” spread by Mr Trump’s supporters, denied categorically having extramarital affairs on Monday.

In a town hall interview with Fox News’ Megyn Kelly, he declared: “I have always been faithful to my wife.”

.@tedcruz: “I have always been faithful to my wife. I love my wife. She is my best friend in the whole world.” #KellyFile

The initial allegations were published last month by the National Enquirer, a supermarket magazine that Mr Cruz has depicted as being owned by a friend of Mr Trump and briefed by one of his aides, Roger Stone.

But as the candidate issued his latest denial, it emerged that the magazine was about to give the rumours fresh impetus by reporting in its next edition that Mr Cruz’s number is believed to appear in the contacts list of the late Deborah Palfrey, who ran an escort agency in Washington offering prostitutes to wealthy clients for $300 (£210) an hour. Palfrey hanged herself in 2008 before she could serve a 55-year jail sentence imposed after a court convicted her of charges that included racketeering and money laundering.

Montgomery Sibley, her former lawyer, has appealed to the United States supreme court for permission to publish the contents of the so-called “black book” - said to contain 15,000 pages of phone records.

Mr Sibley told the National Enquirer that “time is of the essence” in publishing because it has phone numbers containing information “relevant to voters before they cast their ballot”.

A lower court has already barred him from releasing the numbers or naming names.

Investigators have told the National Enquirer that the numbers referred to by Mr Sibley were probably those of Mr Cruz, reported RadarOnline website, which said it had seen a copy of the magazine’s forthcoming report, due to be published on Wednesday.

“If Montgomery Sibley has what he says he has, it has to be Cruz,” said Wayne Madsen, who belonged to a team of reporters who investigated Palfrey’s business for Hustler, a monthly pornographic magazine.

“Going on what I know about the other male candidates still in the race, it isn’t any of them, and Palfrey didn’t provide call boys, so it wasn’t Hillary.”

Dan Moldea, another investigator who examined three of the four boxes of contacts Mr Sibley gave him before the lawyer was subjected to a court gagging order, said he could not recall any other current candidates being on the list.

“I would remember if Bernie Sanders or Trump or John Kasich were in the list,” he told the National Enquirer. “The only name I would not have recognised at the time was Cruz.”

Even if Mr Cruz’s name were in the contacts book, it would not necessarily invalidate his denial of marital infidelity, as the numbers cover the period 1996 to 2006 - which includes a timespan before Mr Cruz married his wife in 2001. It is not clear when his name is said to have featured.

Mr Cruz, who has attempted to woo the Christian evangelical vote, worked in Washington in the late 1990s as a clerk for two supreme court justices. He worked for George W Bush’s presidential campaign team in 2000. Although he was solicitor general in Texas between 2003 and 2008, he only became a nationally known figure after his election to the US senate in 2012.